Except for noble metals such as gold and platinum, metal surfaces generally have a tendency to corrode when exposed to the environment. The corrosion of metals is generally undesirable, because a corroded metal surface may have decreased electrical and thermal conductivity, luster, aesthetic appeal, and mechanical properties such as tensile strength. Billions of U.S. dollars per year are spent protecting, preventing, or retarding corrosion of metals. For example, it is estimated that the direct cost of corrosion to United States industries is about $276 billion annually. When indirect costs are included, such as spillage, production loss, productivity loss, etc., the total rises to $552 billion. See R. Bhaskaran, N. Palaniswamy & N. S. Rengaswamy (2005), “Global Cost of Corrosion—A Historical Review, in Corrosion: Materials, ASM Handbook vol. 13B, p. 619, ASM International, Materials Park Ohio. As a result, there has been great interest in formulating better ways to protect metal surfaces from corrosion. See, e.g., John B. Durkee (September 2006) “The Future of Metal Finishing,” Metal Finishing, vol. 104, no. 9, pp. 60-62.
Different metals corrode in different ways. For example, aluminum, copper, and nickel surfaces usually form an oxide coating after exposure to the environment. The surfaces of other metals, such as alloys of iron, may form rust. The severity of corrosion on metal surfaces depends on the ambient environment and the atmospheric conditions. Although metal surfaces may be exposed to a variety of corrosive environments and chemicals, in most applications, the metal surfaces are exposed to air, which is a source of oxygen and humidity. The oxygen in the air reacts with the metal surface to form the oxide layer, but oxygen alone is not usually sufficient to initiate corrosion in the absence of water. The humidity in the air, however, acts as a carrier of reactive materials, such as dissolved acids, alkali, or salts, to the metal surface, where they contribute to faster oxidation. Once the possibility of water contacting the surface of the metal is reduced, the likelihood of corrosion in air is greatly diminished.
One method of protecting metal surfaces against corrosion is electroplating. One may prevent or retard corrosion by electroplating the metal with a thin layer of non-porous noble metals such as gold or platinum. Protection of the surface may also be achieved by electroplating the surface with such metals as cadmium or chromium, which resist oxidation because of their atomic configuration. Various methods of electroplating are well-known. For example, see Mordechay Schlesinger and Milan Paunovic, eds., Modern Electroplating (4th ed.), Wiley-Interscience Publication, 2000.
Another method for preventing or retarding corrosion of metals is to use a bulk coating technique to brush, spray, or otherwise apply a relatively thick layer of a compound onto a metal surface that adheres to it through physical bonds and prevents the penetration of moisture and/or oxygen. The surface may be coated with an organic compound, which can prevent the penetration of oxygen, and if it is hydrophobic, it can repel moisture as well. Ceramic coatings have also been used in a similar way to cover the metal surface and impede the approach of air and moisture. See, e.g., Yigal Blum & Gregory A. McDermott, Dehydrocoupling Treatment and Hydrosilylation of Silicon-containing Polymers, and Compounds and Articles Produced Thereby, U.S. Pat. No. 5,990,024 (issued Nov. 23, 1999). Various kinds of paint may contain mixtures of organic and/or inorganic compounds that may be coated on surfaces to prevent corrosion. Bulk organic and ceramic coatings are typically several thousandths of an inch thick, and generally convert the metal surface into an electrical insulator.
Another method for preventing or retarding corrosion is to graft organic molecules onto the metal surface, thus creating a thin layer or film that blocks the penetration of oxygen and moisture from the environment. For example, 1,2,3-benzotriazole has been incorporated on a copper surface using an oxidation accelerator for protection against surface oxidation and corrosion. See Xie Hong-Bo & Zhang Lai-Xiang (2006), “High Quality Chromium-Free Passivation Process for Copper and its Alloys”, Journal of Applied Surface Finishing, 1: 94-98. Similarly, nitrophenyl groups have been grafted on metal surfaces via the reactivity of a diazonium group. See Alain Adenier et al., (2005) “Grafting of Nitrophenyl Groups on Carbon and Metallic Surfaces without Electrochemical Induction, Chemistry of Materials, 17:491-501. Grafting organic molecules onto the surface can create a very thin layer, typically a mono-molecular layer, Such a thin layer of coating retains electrical conductivity on the surface of the metal.
Yet another method for reducing surface corrosion is to create nanostructures on the surface that repel droplets of water and prevent them from making significant contact with the surface, thus imparting corrosion resistance. See, e.g., John B. Durkee, (2006) “Are You Ready for Superhydrophobicity?” Metal Finishing, vol. 104, no. 10, pp. 45-47.